1. Field of the Invention
The present invention relates to a method for manufacturing a pad terminal for inputting or outputting electrical signals. More particularly, the present invention relates to a structure of the pad terminal in an electrical circuit board or a liquid crystal display device, and a method of manufacturing of the pad terminal.
2. Discussion of the Related Art
Generally, most electrical circuit boards have pad terminals for inputting or outputting electrical signals. These pad terminals are usually made simultaneously with the rest of the circuit board. Also, an LCD display comprising thin film transistors (TFT's) or other electrical elements has pad terminals similar to ordinary electric circuit boards. The LCD comprises a color filter panel for representing the color of a picture, and an active panel driving a pixel array by applying a data signal corresponding to the picture. These two panels are joined to each other with a narrow gap in between, and a liquid crystal material is injected into the gap. When driving signals are applied to the pad terminals of the active panel, the TFT's of the active panel drive the liquid crystal, thus displaying an image.
The color filter panel comprises a red color filter, a green color filter, and a blue color filter which are arrayed in sequence. A black matrix is located between the color filters in a mesh pattern. A common electrode is formed on the black matrix. The active panel comprises pixel electrodes at the pixels arrayed on the transparent substrate in a matrix pattern. A scan line is formed along the horizontal direction of the pixel array, and a data line is formed along the vertical direction of the pixel array. At one corner of the pixel, a thin film transistor is formed for driving the pixel electrode. The gate electrode is connected to the scan line (sometimes referred to as a `gate line`), and the source electrode is connected to the data line (sometimes referred to as a `source line`). A pad terminal is formed at the end portion of each line.
The scan signal and the data signal are applied to the active panel through the pad terminals. The scan signal is used for selecting the horizontal scan line and has a frequency from 30 Hz to 60 Hz. The data signal normally represents an image. By selecting one scan line using the scan signal, the data signal of the scan line can be displayed an the LCD panel.
The signal applied to the pad terminal is an electrical signal. If there are particles or contaminants between the pad terminal and the output terminal of the outer device, or if the electrical contact between them is poor, then the quality of the image on the LCD screen may degrade. Therefore, the process of forming a pad terminal is very important for maintaining the quality of the image. The conventional method for manufacturing the pad terminal and the structure of the LCD will now be described.
FIG. 1 shows a plan view of the conventional structure of an LCD. FIGS. 2a-2f are cross-sectional views showing the conventional process for manufacturing the TFT, the gate line, and the gate pad of an LCD along the line II--II.
A metal layer having low resistance such as aluminum or an aluminum alloy is deposited on a transparent substrate 11. A gate electrode 13, a gate line 15, a gate pad 17, and a source pad 37 are formed by patterning the metal layer using a photolithographic method, as shown in FIG. 2a. The gate electrode 13 is formed at one corner of the pixel. The gate line 15 connects the gate electrodes 13 arrayed in a row direction. The gate pad 17 is formed later at an end portion of the gate line 15. The source pad 37 is formed at an end portion of a source line 35.
A gate insulation layer 19 is formed by depositing an insulation material such as silicon nitride or silicon oxide on the substrate having the gate electrode 13, the gate line 15, and the gate pad 17. An intrinsic semiconductor material and a doped semiconductor material are sequentially deposited on the gate insulation layer 19. By patterning them, a semiconductor layer 21 and a doped semiconductor layer 23 are formed, as shown in FIG. 2b.
A first gate pad contact hole 61 and a first source pad contact hole 71 are formed by patterning the gate insulation layer 19, as shown in FIG. 2c. The first gate pad contact hole 61 exposes the gate pad 17 by etching the gate insulation layer 19 covering the gate pad 17. The first source pad contact hole 71 exposes the source pad 37 by etching the gate insulation layer 19 covering the source pad 37.
A metal such as chromium or a chromium alloy is deposited on the substrate having the doped semiconductor layer 23. A source electrode 33, a drain electrode 43, a source pad intermediate electrode 77, and a gate pad intermediate electrode 67 are formed by patterning the metal layer, as shown in FIG. 2d. The source electrode 33 contacts one side of the semiconductor layer 21 and the doped semiconductor layer 23, which acts as a source region. The source line 35 connects to the source electrodes 33 arrayed in a column direction. The drain electrode 43 is on the other side of the doped semiconductor layer 23 from the source electrode 33, and connects to the other side of the semiconductor layer 21 and the doped semiconductor layer 23, which acts as a drain region. The source pad intermediate electrode 77 is formed at the end of the source line 35 and connects to the source pad 37 through the first source pad contact hole 71. The gate pad intermediate electrode 67 connects to the gate pad 17 through the first gate to pad contact hole 61.
A protection layer 39 is formed by depositing an insulation material such as silicon oxide or silicon nitride on the substrate having the source electrode 33. A drain contact hole 81, a second gate pad contact hole 87, and a second source pad contact hole 97 are formed by patterning the protection layer 39, as shown in FIG. 2e. The drain contact hole 81 exposes the drain electrode 43 by removing a portion of the protection layer 39 covering the drain electrode 43. The second gate pad contact hole 87 exposes the gate pad intermediate electrode 67 by removing a portion of the protection layer 39 covering the gate intermediate electrode 67. The source pad contact hole exposes the source intermediate electrode 77 by removing a portion of the protection layer 39 covering the source intermediate electrode 77.
A pixel electrode 53, a gate pad terminal 65, and a source pad terminal 75 are formed by depositing and patterning indium tin oxide (ITO) on the protection layer 39, as shown in FIG. 2f. The pixel electrode 53 connects to the drain electrode 43 through the drain contact hole 81. The gate pad terminal 65 connects to the gate pad intermediate terminal 67 through the second gate pad contact hole 87. The source pad terminal 75 connects to the source pad intermediate electrode 77 through the second source pad contact hole 97.
According to the conventional method, the gate pad 17 and the source pad 37 comprise aluminum, the gate and source pad intermediate electrodes 67 and 77 comprise chromium, and the gate source pad terminals 65 and 75 comprise ITO. Generally, when ITO is deposited by a sputtering method on the intermediate electrodes 67 and 77, it is sputtered in an oxygen atmosphere. At that time, chromium oxide can form on the surface of the intermediate electrodes 67 and 77. Also, the gate insulation layer or the protection layer comprising the silicon oxide or silicon nitride are formed on the pads or on the intermediate electrodes and patterned to form contact holes. When silicon oxide or silicon nitride is deposited on the surface of the metal layer, a thin metal oxide layer or a thin metal nitride layer can form, resulting in a contact resistance between chromium and ITO that is higher than desired.